Laser interference photoetching is an important method of manufacturing micro or nano-scale array device by exposing a photosensitive substrate using periodical diagram generated by interference between two or more laser beams. This photoetching technology is mainly applied in manufacturing devices which has a feature size lower than subwavelength such as hole array/matrix, spot array/matrix, pillar array/matrix, grating and micro-lens array etc., which have a wide application in national defense, civil engineering and scientific research etc.
Recently, some important engineering systems such as large astronomical telescope, laser ignition system for inertial confinement nuclear fusion and photoetching system etc., have an increasing demanded for size, grid density and precision, and grating manufacturing is developing towards meter-scaled size, nano-scaled precision and grid density of sub-10^4, and manufacturing gratings which have large size, high precision and high grid density has becoming a problem of main concern which needs to be solved. Conventional manufacturing technique mainly includes mechanical scratching, laser direct writing, and mechanical splicing, etc. The mechanical scratching has the disadvantages of poor precision in large-size processing, long periodical of processing, and ghost line present in processed gratings etc. The laser direct writing has the disadvantages of poor precision in large-size processing and long periodical of processing etc. The mechanical splicing has the disadvantages of poor precision of splicing, complicated splicing process and high cost etc. Thus it is difficult to use these techniques to achieve the above mentioned quality. In contrast, laser interference photoetching or holographic photoetching has advantages of high grid density and high precision in large-size process and short processing cycle etc. in manufacturing grating of the above-mentioned high quality, therefore, laser interference photoetching is gradually becoming the mainstream technology in manufacturing grating of large-size and high precision. The development of laser interference photoetching system is a major difficulty in laser interference photoetching applicable in manufacturing grating of large-size and high precision, and the development of laser interference photoetching system of high precision is really high technology. With respect to the development of laser interference photoetching system of high precision, major large companies or corporations and research institutes in the art have exerted lots of efforts. Image locking of high speed and high precision is a key technology which directly determines the precision of image locking. Many scientific institutes have made developments in this field, some of which have been disclosed in patents.
One patent of MIT, i.e., U.S. Pat. No. 5,142,385 discloses a laser interference photoetching system which comprises a image locking device which performs image locking through phase modulation of beams with an electro-optic phase modulator, but the electro-optic phase modulator has the disadvantages of low speed of phase modulation, low precision of modulation and narrow range of modulation, which is difficult to meet the requirements for image locking of high precision.
Another patent of MIT, i.e., U.S. Pat. No. 6,882,477B1 discloses a scanning laser interference photoetching system, which comprises an acoustic-optic image locking device which performs image locking through phase modulation of beams with an acoustic-optic modulator. Although the acoustic-optic modulator has advantages of high speed of modulation, high precision of modulation and infinite range of modulation etc., however, the acoustic-optic modulator performs phase modulation through frequency modulation, which changes the frequency and coherence characteristics. And during the frequency modulation, the direction of beam emitted from the acoustic-optic modulator also changes which may influence the precision of image locking, the contrast ratio of image, and depth of focus, and size of focal spot etc. of the system. Besides, the phase modulation by the acoustic-optic modulator may increase the complicatedness of optical path of the system. Taking the above issues into consideration, the phase modulation by the acoustic-optic modulator may not be an optimal choice. Meanwhile, this patent also discloses an image locking device which performs image locking through phase modulation with an optical reflector seat driven by piezoelectric ceramics. This image locking device performs phase modulation of beams by changing the position of the reflector with the optical reflector seat driven by piezoelectric ceramics. This method does not change the frequency of beams and does not influence the coherence characteristics of the exposing rays, but the change of the position of the reflector may influence the contrast ratio of image, and depth of focus and size of focal spot etc. of the system, and the driving of the optical reflector seat by piezoelectric ceramics has the disadvantages of low speed of modulation, low precision of modulation, and limited range of modulation, which is also difficult to meet the requirements of image locking of high speed and high precision.
In view of the above problems, the present invention aims to provide an optical grating phase modulator used for phase modulation in a laser interference photoetching system, in which the phase modulator is achieved by Doppler shift effect of a moving grating, and such a phase modulator has the advantages of high speed of phase modulation, high precision of modulation and wide range of modulation etc. With a proper arrangement of laser interference photoetching system, the phase modulation of the grating phase modulator does not influence the precision of the system, the contrast ratio of image, and depth of focus and size of focal spot etc. of the system, and besides, the optical path of the system is simple, all of which make contributions to the improvement of the overall performance of the laser interference photoetching system.